Conventionally, leather-like sheets containing a skin-like layer (also commonly referred to as a grain side layer) are used in a multitude of applications, including footwear, clothing, bags, and furniture and the like.
The basic construction of a leather-like sheet is a laminated structure including a skin-like layer, an adhesive layer, and a fibrous substrate. Examples of suitable materials for the fibrous substrate include base fabrics such as nonwoven, woven and knitted fabrics, materials in which a polyurethane resin or the like has been impregnated into any of these base fabrics, and materials in which a porous layer containing a resin such as a polyurethane resin has been provided on the surface of one of these substrates.
A number of processes for producing leather-like sheets are already known. Representative examples of these processes include (1) a process in which an organic solvent solution of a polyurethane resin that has been colored using any of a variety of colorants is applied, by gravure coating, to the surface of an aforementioned fibrous substrate on which has been provided a porous surface layer containing a resin such as a polyurethane resin, and following removal of the organic solvent by drying, an embossing roll or the like is pressed onto the surface of the applied coating, thereby forming a skin-like layer with a leather-like indentation pattern (leather-like uneven pattern), and (2) a process in which an organic solvent solution of a colored polyurethane resin is applied to the surface of a release paper with a leather-like indentation pattern, the organic solvent is removed by drying, and the resulting skin-like layer is then bonded to a fibrous substrate using an adhesive.
In all of the above types of conventional processes for producing leather-like sheets, an organic solvent solution of a polyurethane resin is used. As a result, removal of the organic solvent by drying or extraction is essential at some point during the production steps. This raises a number of problems, including the deleterious effect of organic solvents on human health, pollution of the atmosphere or waterways, and the large energy requirements and production costs associated with evaporating the organic solvent, and as a result, the development of processes that use either water-based polyurethane resins or solvent-free polyurethane resins, with no requirement for the use of organic solvents, has been keenly sought by industry.
Furthermore, depending on the application, there are also demands for high levels of abrasion resistance, or deeper degrees of coloring, and in order to meet these demands, skin-like layers with increased film thickness may be required. However, in a conventional process wherein following application of an organic solvent solution of a polyurethane resin, the skin-like layer is formed by removal of the organic solvent by drying, the maximum film thickness achievable using a single application is only approximately 150 μm. Accordingly, a plurality of application repetitions is required, but this leads to further increases in the discharge quantity of organic solvent, the energy requirements, and the production costs.
One process that is being investigated as a potential countermeasure to the above problems is a process that uses a water-based polyurethane resin instead of an organic solvent-based polyurethane resin. However, the leather-like sheets obtained using a water-based polyurethane resin generally exhibit inferior water resistance and durability, meaning their practical applicability is poor. Furthermore, in those cases where a skin-like layer with increased film thickness is required, a plurality of application repetitions is required, and as a result, the number of production steps and the costs are still problematic, meaning the range of applications to which this process can be applied is extremely limited.
Furthermore, a process is also known wherein a thermoplastic polyurethane with a specific composition is applied to the surface of a release paper with a leather-like indentation pattern using melt extrusion, and the thermoplastic polyurethane film layer on top of the release paper is then transferred and bonded to a fibrous substrate using a compression roller or the like (for example, see patent reference 1). In the case of this process, high temperature conditions of 180 to 230° C. are required to melt the thermoplastic polyurethane, and if the thermoplastic polyurethane resin is colored in advance using a colorant such as a pigment, then the heat during the melt extrusion can cause discoloration of the colorant. Furthermore, because the temperature of the molten-state thermoplastic polyurethane resin is very high, at 180° C. or higher, the thermoplastic polyurethane resin undergoes rapid cooling when applied to the release paper, meaning the polyurethane may not penetrate into some of the fine indentations (irregularities, ruggedness) within the release paper, resulting in a poor reproduction of the indentation pattern. In addition, because of the high temperature, oxidation decomposition and hydrolysis of the molten thermoplastic polyurethane becomes more likely, and in order to prevent these reactions, large-scale apparatus must be used to exclude oxygen and moisture.
Furthermore, a technique that uses a moisture-curable polyurethane holt-melt resin composition is also being investigated as a potential production process that uses a solvent-free polyurethane resin. For example, a process for producing a polyurethane leather-like sheet has been disclosed in which a solid moisture-curable polyurethane (a moisture-curable polyurethane hot-melt resin composition) can be used as the skin-like layer for a leather-like sheet, and this skin-like layer and a substrate are then bonded together using the moisture-curable polyurethane as an adhesive (for example, see patent reference 2).
In this process, steps are required for preparing the skin-like layer in advance, and then bonding the skin-like layer and the substrate together using the moisture-curable polyurethane hot-melt resin composition as an adhesive, and consequently the production process is complex. Furthermore, leather-like sheets are usually colored, meaning in order to use the moisture-curable polyurethane hot-melt resin composition as the skin-like layer, this composition must be colored in some manner. However, coloring a moisture-curable polyurethane hot-melt resin composition in such a manner that a favorable external appearance is obtained is far from simple. Only limited knowledge is available concerning such coloring methods, and the references mentioned above contain absolutely no comment on a coloring method.
Patent Reference 1:
Japanese Unexamined Patent Application, First Publication No. Hei 9-24590
Patent Reference 2:
Japanese Unexamined Patent Application, First Publication No. 2000-54272